The invention may generally be applied to the phase synchronization of step-drive controlled equipment to signals or to other equipment.
The invention is especially adapted for use in optical modulation devices for multiple-beam spectrophotometers driven by stepping motors.
Devices running continuously, in which signals are generated that are synchronous to operation, are known in many cases. For instance, evaluation of modulation devices, in particular for multiple-beam spectrophotometers, are required regardless of whether the devices are driven, as hitherto generally customary, by synchronous motors (for example, DD-PS No. 65,468) or stepping motors (for example, DE-OS 3,202,807, DD-PS No. 228,058). In DE-OS No. 3,202,807 a high-frequency clock signal is used to generate the control frequency of the step drive therefrom with a divider and, in addition, to form a demodulation signal via an additional divider, synchronized by the optical signal by means of zero crossing detectors.
This solution, which is applicable only to single-beam instruments, additionally has the considerable disadvantage that, at high absorptions of the optical sample, noise causes the demodulation to deteriorate.
In DD-PS No. 228,058 pulses of step-drive control frequency are supplied to a step drive, but are alternatively guided through an adjustable retarding step with subsequent pulse narrowing and activate a counter logic circuit so that the evaluation signals thereby formed, which serve for analog data processing, have the proper phase with respect to the electrical analog signals formed from the optical signals.
For this purpose an optocoupler cooperating with the modulation mirror delivers a synchronization signal which resets the counter logic circuit at the beginning of each modulation period. Although the disadvantages of DE-OS No. 3,202,807 are thereby avoided, since the resettable retarding step permits adjustment to be made only within a step, it is necessary to preadjust the modulation mirror and optocoupler with stepping motors excited in definite fashion or to incorporate them in a device with sufficient accuracy.
In addition to the expense in manufacture of the instrument, the necessary preliminary adjustment complicates service on the modulation device, including the optocoupler.
Further, in the event, for example, of aging phenomena with varying load moment on the stepping motor, phase shifts between optical signals and evaluation signals appear and, with them, errors of measurement, which require a manual readjustment.
It has already been proposed (WP G No. 01 J/282,311) that for synchronization, referred to a predetermined desired phase signal, by control frequencies differing from the synchronous frequency, the stepping motor may run one step per cycle slower or faster until desired-phase and sensor signals coincide sufficiently accurately. Here the same disadvantages appear as in DD-PS No. 228,058.
In addition, it is generally known that a step drive may be activated not by pulses of a control frequency, but that commutator signals, with which the windings of the stepping motor are connected, may be generated by, for example, a computer.